Explanation:

The OSI (Open Systems Interconnection) model is a conceptual framework for understanding and describing how different network protocols and technologies interact and work together. It consists of seven layers, each with its own specific functions and responsibilities. The layers are as follows:

1. Application Layer:
The application layer is responsible for providing services directly to the end-user applications. It enables users to access network resources and services such as web browsing, email, file transfer, and remote login. It defines the protocols and methods used by applications to communicate with each other.

2. Presentation Layer:
The presentation layer is responsible for the representation and transformation of data to ensure that applications on different systems can understand each other. It handles tasks such as data encryption, compression, and formatting. It also takes care of data syntax and semantics, ensuring that the data is properly formatted and compatible with the receiving application.

3. Session Layer:
The session layer establishes, manages, and terminates connections between applications on different devices. It handles session setup, synchronization, and teardown, ensuring that the communication between applications is reliable and secure. It also provides services such as checkpointing, which allows a session to be restored in the event of a failure.

Additional Layer:

The OSI model typically consists of seven layers, but in some versions or interpretations of the model, an additional layer called the session layer is included. This additional layer is placed between the presentation and transport layers.

The session layer is responsible for managing and coordinating communication sessions between applications. It provides services such as session establishment, synchronization, and termination. It ensures that data is exchanged in an orderly manner and that both the sending and receiving applications are ready to receive and process the data.

Conclusion:

In summary, the OSI model consists of seven layers, including the application layer, presentation layer, and an additional session layer. The session layer is responsible for managing and coordinating communication sessions between applications. Including the session layer in the model allows for a more detailed and comprehensive understanding of the communication process between applications.